The present invention relates to steam turbines and, more particularly, to a method of controlling operation of a steam turbine bypass system in conjunction with steam boiler control to prevent rapid temperature excursions in response to sudden drops in load demand.
In a typical steam turbine power plant, a steam generator such as a boiler produces steam which is provided to a high pressure (HP) turbine through a plurality of steam admission valves. Steam exiting the high pressure turbine is reheated in a conventional reheater prior to being supplied to a lower pressure turbine, the exhaust from which is conducted into a condenser where the exhaust steam is converted to water and supplied to the boiler to complete the cycle. A typical power utility system will employ one or more HP turbines, an intermediate pressure (IP) turbine and a low pressure (LP) turbine. The turbines are generally coupled to drive a synchronous electric power generator at constant speed for producing electric utility power which is transmitted over a transmission link to various users.
With steam turbines equipped with a steam bypass system, the steam admission valves to the turbine may be closed, or partially closed, while still allowing steam to be produced by the boiler at a load level independent of steam turbine load by directing the excess steam through the bypass system to the condenser. The bypass system is advantageously used for hot restarts or to keep the boiler on-line during plant or system transients that would normally require a trip (shutdown),i.e., a sudden loss of load demand such as might be caused by a loss or interruption on the power transmission link. Accordingly, bypass systems are provided in order to enhance on-line availability, obtain quick restarts, and minimize turbine thermal cycle expenditures.
In the event of a sudden drop in load demand, it is still desirable to operate the steam turbine system at a house load level so as to supply the electrical needs of auxiliary equipment such as pumps, pulverizers, fans, etc. Under such conditions, the turbine will continue running at synchronous speed although with a greatly reduced steam flow and with the remainder of the boiler-produced steam being provided to the bypass system.
Ordinarily, sufficient steam flow must be passed through the turbine in order to keep the turbine elements cool. With the reduced flow rate conditions, however, a windage effect takes place whereby instead of extracting work from the steam, the turbine blades are actually doing work on the steam which is being churned up, resulting in a temperature increase which in turn causes the turbine parts to heat up. A danger therefore exists under such conditions, of turbine overheating past its design rating, thereby resulting in reduced life and possible premature failure. U.S. Pat. No. 4,576,008 describes an improved bypass system which provides a sufficiently low pressure at the HP turbine exhaust so as to maintain the temperature of the turbine components within design limits.
For a typical reheat turbine bypass system, all but a small fraction of the main steam flow is diverted around the HP element and is directed to a pressure reducing and desuperheating station. The pressure is reduced to a level that corresponds to the HP exhaust (cold reheat) pressure that existed prior to the load interruption. The HP exhaust pressure level is maintained by the combined action of additional bypass valves at the reheater outlet and modulating interceptor valves at the IP section inlet. The major portion of the steam flow is directed to the condenser by the bypass valves at the hot reheat outlet. Sufficient flow is passed through the HP, IP and LP sections to meet the auxiliary load of the plant.
When the electrical connection with the load center is restored, the flow through the bypass system is reduced and the turbine section flows are correspondingly increased to increase the generator output. There is no time lag introduced by the boiler as its steaming capability has been maintained at the level prior to the load center loss. When the duration of the load loss is longer than some predetermined interval, the boiler heat input is reduced with a concurrent reduction in the bypass flow and the auxiliary power requirements. During the change to a lower level of boiler input energy, the HP exhaust pressure and consequently the cold and hot reheat pressures are reduced to a level consistent with the lower main steam flow. The main steam flow (and consequently the boiler heat input) are reduced to a level consistent with extended low load operation and this also reduces the energy consumption of the plant.
When the bypass system is initially activated with a conventional bypass system in which boiler steaming capacity is maintained, there is a rapid increase in HP section exhaust temperature and a rapid decrease in HP section inlet blading temperature. If the HP section has a partial-arc admission first stage, the first stage exit temperature decreases even more than the first stage inlet temperature, possibly by more than 200.degree. F. depending upon the active arc of admission.
Because of the large decrease in HP turbine blading inlet flow, the control valves must throttle. Steam, not being a perfect gas, decreases in temperature when it is throttled. For example, in going from full load to house load, the steam flow is reduced by a factor of about 12, which means that the control valve discharge pressure is about 8% of the full load pressure. In the case of 2400 psia, 1000.degree. F. steam, its temperature decreases by about 120.degree. F. For 3500 psia, 1000.degree. F. steam, the temperature change is about 180.degree. F. So the HP first stage inlet temperature would be 120.degree. F. to 180.degree. F. lower than the full load temperature immediately after the bypass system is activated. As a result, there is a large thermal transient imposed upon the control valve body, inlet piping, HP inner shell inlet (nozzle chambers if they are present), and HP rotor.
If the first stage is a full-arc or fixed-arc admission stage, its exit temperature would decrease by about the same amount as its inlet temperature. In the case of a typical Rateau type fixed-arc admission stage, the difference between the stage inlet and outlet temperatures would be about 65.degree. F. If the first stage were a partial-arc admission Rateau stage with 50% minimum admission, the first stage exit temperature would decrease by an additional 105.degree. F. So the total change in the first stage exit temperature is the temperature change resulting from the throttling and the 105.degree. F. blading induced change. Consequently, for a 2400 psia turbine, the total decrease in first stage exit temperature is 225.degree. F. (120.degree. F. plus 105.degree. F.) and is 285.degree. F. (180.degree. F. plus 105.degree.) for 3500 psia steam.
The majority of the downstream stages in the HP section do work on the steam because of windage heating (low flow and elevated HP exhaust pressure). As a result, the steam temperature will increase from stage to stage to a level where it may not be tolerable. The increase in HP exhaust temperature also causes thermal stress at the exhaust. The above mentioned U.S. Pat. No. 4,576,008 "Turbine Protection System for Bypass Operation" presents a solution to the HP exhaust temperature change.